Trace analyte detection has numerous applications, such as screening individuals and baggage at transportation centers, mail screening, facility security applications, military applications, forensics applications, narcotics detection and identification, cleaning validation, quality control, and raw material identification. Trace analyte detection is the detection of small amounts of analytes, often at nanogram to picogram levels. Trace analyte detection can be particularly useful for security applications such as screening individuals or items for components in explosive materials, narcotics, chemical substances, or biological contaminants where small amounts of these components are deposited on the individual or on the surface of a package or bag.
A variety of different techniques can be used for trace analyte detection. These methods include ion mobility spectrometry (IMS), mass spectrometry, gas chromatography, liquid chromatography, and high performance liquid chromatography (HPLC).
IMS is a particularly useful technique for rapid and accurate detection and identification of trace analytes such as narcotics, explosives, and chemical warfare agents. The fundamental design and operation of an ion mobility spectrometer is addressed in, for example, Ion Mobility Spectrometry (G. Eiceman and Z. Karpas, 2d Ed., CRC Press, Boca Raton, Fla., 2004). IMS detects and identifies known analytes by detecting a signal which is unique for each analyte. IMS measures the drift time of ions through a fluid, such as clean, dry ambient air at atmospheric pressure. Analysis of analytes in a sample begins with collection of a sample and introduction of the sample into the spectrometer. A sample is heated to transform analyte from solid, liquid or vapor preconcentrated on a particle into a gaseous state. Analyte molecules are ionized in the reaction region of the IM spectrometer. Ions are then spatially separated in the IMS drift region in accordance to their ion mobility, which is an intrinsic property of an ion. Often, an induced current at the collector generates a signature for each ion as a function of the time required for that ion to reach the collector. This signature can be used to identify a specific analyte.
An advantage of using IMS for trace detection is the ability to analyze a sample in both positive and negative mode and using different ionization reagents to identify substances that cannot be differentiated by other methods. For example, ranitidine and cocaine have similar mobility constants in the positive mode. However, only ranitidine is ionized in the negative ion mode, allowing differentiation of ranitidine and cocaine when the positive and negative mode data both are collected and analyzed. Additionally, ammonium nitrate can be difficult to distinguish from other analytes containing ammonium ions or nitrate ions, but can be differentiated when the results from both positive and negative mode ionization are analyzed.
Trace detection can be useful for detecting analytes on documents. For example, travel documents, such as passports and visas, may contain analytes because a traveler has handled illicit or illegal substances before handling the travel documents. Conventional detection devices require an operator to collect a sample from a surface of a targeted document with a sample collection device, such as by swabbing or wiping the surface, and then inserting the sample collection device into a detection instrument for analysis. Such a procedure requires multiple steps, which slows the detection process.